1. Field of the Invention
This invention relates to an adhesive film and a flat cable using the adhesive film.
2. Description of the Related Art
Flat cables generally refer to cables with a plurality of parallel arranged rectangular conductors sandwiched between two films each having an adhesive layer (herein referred to as “adhesive films”), and are thin in thickness and feature an excellent flexibility. These flat cables have, because of their advantageous flexibility feature, been being widely used as internal wiring cables for various electric and electronic equipment, such as office automation equipment of printers, scanners, etc., computer equipment, video equipment of flat televisions, etc., audio equipment, robots, ultrasonic diagnostic equipment, and the like. Further, in recent years, because of increasing use of electronics in automobiles, the flat cables have become widespread for use on vehicles as well.
To use the flat cables as the internal wiring materials for electronic equipment among others, the flat cables must satisfy UL (Underwriters Laboratories Inc.) standards, and are required to have a high flame retardancy. A solution to this has been to make the adhesive films covering the conductors of the flat cables flame retardant. The adhesive films covering the conductors of the flat cables have been produced by, in many cases, applying a wet coat of adhesive dissolved in a solvent to an insulating film base material to form an adhesive layer.
The insulating film has used an engineering plastic film which is excellent in heat resistance and chemical resistance. Among other engineering plastic films, a polyethylene terephthalate (PET) film has generally been being used because of being widely commercially available in significant quantities, and excellent in cost and stability of supply. Incidentally, in order to enhance the adhesion between the PET film and the adhesive, the PET film is used by corona treating or ultraviolet treating its side to be coated with the adhesive.
There have been two methods to make the flat cables flame retardant, one by making the insulating film itself flame retardant, and the other by making the adhesive flame retardant.
As the method by making the insulating film itself flame retardant, there has been a method by using a film made of a self-extinguishing resin such as a polyimide resin. This self-extinguishing resin film has been very expensive however, and has been being used only for specific purposes. Therefore, a method by adding a flame retardant to the adhesive has generally been being employed.
As the base resin for forming the adhesive layer of the adhesive films, a thermoplastic polyester resin which provides an especially good adhesion to the polyethylene terephthalate film has been being widely used. As the thermoplastic polyester resins, there are amorphous and crystalline resins. The amorphous resin dissolves well in a versatile organic solvent, and therefore a wet coating thereof has been produced and has been being widely used as the resin for forming the adhesive layer of the general purpose flat cables. There has been the problem, however, that the amorphous resin is low in heat resistance, and therefore cannot be used for heat resistance.
Therefore, as a method by making the amorphous resin heat resistant, there has been a method by adding a curing agent to introduce a cross-link structure. This heat resistance enhancing method by introducing the amorphous polyester cross-link structure has been not likely to have a great effect in comparison to that by introducing the crystalline resin. Also, excessively introducing the cross-link structure is not likely to allow hot melting of the adhesive layer in flat cable production, and may therefore not result in a sufficient adhesive force of the adhesive layer.
On the other hand, the crystalline resin has a good heat resistance, and can be used as the resin for forming the adhesive layer of the flat cables used for heat resistance. However, the crystalline polyester resin tends to be insoluble in the solvent due to its crystallinity, and substantially does not dissolve in the versatile organic solvent. Therefore, there has been considered an adhesive layer forming method with a wet coating produced by dissolving the crystalline polyester resin in a chlorine based organic solvent with an especially high solvent power, such as a methylene chloride or the like. However, the chlorine based organic solvent may have a harmful effect on the human body and the environment, and its use therefore tends to be reduced.
Further, in order to use the crystalline polyester resin, there has also been considered a flat cable producing method by thin extrusion with an extruder.
Refer to JP-A-2002-367458, for example.